Spinning drop approach

In many emulsion or microemulsion systems, the interfacial tension between the oil-rich phase and the aqueous solution is very low (or ultralow), which presents considerable difficulties for many experimental methodologies. The most commonly employed approach for measuring ultralow interfaeial tension is the spinning-drop technique (98). However, ADSA has also been used to study these systems and possesses a number of advantages over the... 

In the present work, we summarize recent observations on the coalescence behavior of several oil-water-surfactant systems. We are particularly interested in the behavior exhibited by low tension systems and the role of bulk phase viscous effects and dynamic interfacial properties such as interfacial shear and dilatational viscosities. Also, a new experimental approach for conducting coalescence studies is described which involves tests in an inclined spinning drop device. This approach allows for coalescence tests under controlled conditions and provides an efficient method for quickly screening and evaluating different surfactant systems. 

There are many methods available for the measurement of surface and interfacial tensions. Details of these experimental techniques and their limitations are available in several good reviews [101-104]. Table 5 shows some of the methods that are used in petroleum recovery process research. A particular requirement of reservoir oil recovery process research is that measurements be made under actual reservoir conditions of temperature and pressure. The pendant and sessile drop methods are the most commonly nsed where high temperatur pressure conditions are required. Examples are discussed by McCaffery [i05] and DePhUippis et al. [J06]. These standard techniques can be difficult to apply to the measurement of extremely low interfacial tensions (< 1 to 10 mN/m). For ultra-low tensions two approaches are being used. For moderate temperatures and low pressures the most common method is that of the spinning drop, especially for microemulsion research [107], For elevated temperatures and pressures a captive drop method has been developed by Schramm et al. [JOS], which can measure tensions as low as 0.001 mN/m at up to 200 °C and 10,000 psi. In aU surface and interfacial tension work it should be appreciated that when solutions, rather than pure liquids, are involved appreciable changes can occur with time at the surfaces and interfaces, so that techniques capable of dynamic measurements tend to be the most useful. 

The absolute values of the interfacial tensions varied between different amphi-philes and solvents (Table 1). AOT, which is well known in the literature for the formation of microemulsions, showed the lowest surface tension at the interface of both solvents. The other nonionic snrfactants mentioned here. Span 80 and Brij 72 showed shghtly higher valnes. This was also observed for Lecithine, but this lipid precipitated partly during the spinning-drop measurements. Due to this phenomenon, it was not possible to measure accurate data for this emulsifying compound. The interfacial tension had also some influence on the mean size of the emulsion droplets and on the stability of the vesicles (Table 3). In addition to the stationary values of the surface tension, dynamic processes as the surfactant diffusion represented another important factor for the process of stimulated vesicle formation. If an aqueous droplet passed across the fluid interface it carried-over a thin layer of emulsifiers and thereby lowered the local surfactant concentration in the vicinity of the oil-water interface. In the short time span, before the next water droplet approached the interface, the surfactant films should entirely reform and this only occurred, if the surfactant diffusion was fast enough. 

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